1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for testing a liquid crystal display device.
2. Description of the Prior Art
In recent years, liquid crystal display devices, which are lightweight, space-saving, and power-saving, have been in greatly increasing demand for their compactness and portability, and have been employed in large quantities in desk-top monitors, as displays in portable electronic devices, and in many other applications.
FIG. 7 is a perspective view showing the structure of a liquid crystal display device of an active-matrix type. In the following description, among many types of liquid crystal display device, an active-matrix type using thin-film transistors (hereinafter abbreviated to TFTs) or the like will be taken up as an example.
On a glass substrate 20, scanning signal lines 21 and data signal lines 22 are formed so as to cross each other and, at their crossings, TFTs 23 and pixel electrodes 24 are formed. Each TFT 23 has its gate connected to one of the scanning signal lines 21, has its source connected to one of the data signal lines 22, and has its drain connected to one of the pixel electrodes 24. Usually, a plurality of sets of these elements are arranged in an array that extends two-dimensionally. Above the glass substrate 20 on which these elements are formed, a glass substrate 26 having a transparent electrode 25 is provided so as to face the glass substrate 20, and liquid crystal 27 is sealed in between the two substrates 20 and 26. In the figure, the TFTs 23 are illustrated in a simplified manner.
Color display is achieved by providing color filters 28, 29, and 30 of three different colors, i.e. red, green, and blue (hereinafter abbreviated to R, G, and B colors, respectively), on those portions of the transparent electrode 25 that face the pixel electrodes 24 in such a way that the three colors recur every three lines. Usually, three spots that respectively give off R, G, and B colors together form one pixel. On the outer surfaces of the glass substrates 20 and 26 are provided, though not shown, polarizing plates in the form of films.
In the liquid crystal display device shown in FIG. 7, when a voltage is applied between the pixel electrodes 24, which are provided on the glass substrate 20, and the transparent electrodes 25, which are provided on the glass substrate 26, the liquid crystal molecules stand upright, which hinders light from being transmitted and thus turns the screen black. When no voltage is applied, the screen is white, and therefore this type of liquid crystal display device is called a xe2x80x9cnormally whitexe2x80x9d type. In FIG. 7, whereas the liquid crystal molecules are in twisted orientations for the R and G colors, permitting light to be transmitted through the corresponding color filters 28 and 29, no light is transmitted for the B color. Here, the voltage is applied only between the electrodes for the B color. This can be controlled by varying the voltage that is fed from the data signal lines (22 for the B color) through the TFTs to the pixel electrodes for the individual colors.
In a liquid crystal display device as described above, if variations occur in the thickness and characteristics of the color filters when they are formed or laid in position, such variations cause uneven display of colors. To inspect for such uneven display of colors, for example to inspect for uneven display of the R color, a test is conducted with the liquid crystal display device fed with a test signal that makes it transmit light only through the electrodes for the R light. Specifically, test probes are put in contact with the terminals of the data signal lines, and a driving voltage is applied to the electrodes for the G and B colors so that the R color is displayed. Similarly, in a liquid crystal display device, tests for short circuits between the pixel electrodes, short circuits between the data signal lines, and other defects are performed with probes put in contact with the data signal lines.
However, as higher and higher resolution is sought in liquid crystal display devices, they come to have increasing numbers of pixels and increasingly small pixel pitches. This requires that their testing equipment be provided with increasing numbers of probes arranged at increasingly short intervals, which complicates the design of the testing equipment and makes it expensive. Moreover, inconveniently, very short intervals between the probes tend to cause unwanted contact between the probes themselves.
To avoid this, Japanese Patent Application Laid-Open No. H7-5481 proposes a method for testing the display of colors. According to this method, along three lines substantially perpendicular to the data signal lines, the data signal lines are coated with an insulating film with every third data signal line, of which the position is shifted from one perpendicular line to the next, left exposed. Then, simply by putting three conductive short-circuiting bars in contact with the data signal lines along the three lines perpendicular thereto, it is possible to feed different electric signals to the data signal lines for different, i.e. R, G, and B, colors through the exposed portions thereof, and thereby test the display of colors.
However, this method requires the formation of the insulating film that permits selective contact between the three short-circuiting bars and the data signal lines, and thus requires an extra area outside the display area. That is, inconveniently, the liquid crystal display device as a whole needs to be made larger just to secure an area that is used only in testing and is thus of no use in actual use.
An object of the present invention is to provide a liquid crystal display device that permits easy testing for uneven display of colors, short circuits, and other defects, and to provide a method for testing a liquid crystal display device for such defects.
To achieve the above object, according to one aspect of the present invention, a liquid crystal display device is provided with data signal lines consisting of recurrently formed first, second, and third lines that have open ends and are used for data entry, first diodes formed on the first lines, and second diodes formed on the second lines and having the opposite polarity to the first diodes. Here, the data signal lines are so formed as to permit a short-circuiting bar for supplying testing voltages to be put in contact with the data signal lines at the portions thereof nearer to the ends thereof than the first and second diodes.
With this structure, simply by applying a single DC (direct-current) voltage by way of a short-circuiting bar, which promises easy and secure contact, it is possible to display the desired color according to the presence/absence and the polarity of the diodes. In this way, it is possible to realize easily a liquid crystal display device that permits easy testing for uneven display of colors. Thus, when the liquid crystal display device is tested, there is no need to put probes in contact with all the data signal terminals to apply testing voltages thereto, and thus there is no risk of the test being conducted erroneously as a result of wrong contact between the fine probes and the data signal terminals, short-circuits between the probes themselves, which are arranged at very small intervals, or other causes. Here, the area additionally needed for the testing can be secured without unduly increasing the area of the liquid crystal display device.
When the liquid crystal display device is tested, the short-circuiting bar may be connected to the data signal lines at the portions thereof nearer to the display area than the diodes. This eliminates the need to put probes in contact with all the data signal terminals to apply testing voltages thereto, and thereby helps realize a liquid crystal display device that permits testing for uneven display of halftone between white and black and other defects by the use of a short-circuiting bar, which promises easy and secure contact.
Moreover, it is possible to realize a liquid crystal display device that permits easy testing for uneven display of colors by permitting one of the red, green, and blue colors to be displayed at one time according to the presence/absence and the polarity of the diodes even when the short-circuiting bar is put in contact with all of the first, second, and third lines.
According to another aspect of the present invention, a method for testing a liquid crystal display device provided with data signal lines consisting of recurrently formed first, second, and third lines that have open ends and are used for data entry, first diodes formed on the first lines, and second diodes formed on the second lines and having the opposite polarity to the first diodes includes a step of putting a short-circuiting bar for supplying testing voltages in contact with the data signal lines at the portions thereof nearer to the ends thereof than the first and second diodes and a step of applying different direct-current voltages sequentially to the short-circuiting bar.
By this method, it is possible to display colors in different ways according to the presence/absence and the polarity of the diodes simply by applying a single DC voltage by the use of a short-circuiting bar, which promises easy and secure contact, and thereby conduct a test for uneven display. Thus, when the liquid crystal display device is tested for uneven display of colors, there is no need to put probes in contact with all the data signal terminals to apply predetermined testing voltages thereto. This helps reduce the risk of erroneous testing and thereby greatly reduce the testing cost.